Car advertisements should explain that official fuel consumption figures "may not reflect real driving" according to a new ruling by the Advertising Standards Authority (ASA).

In what is expected to be a landmark case, a car buyer complained to the ASA that the Audi A3 1.6 TDI they had bought did not match the 68.9mpg EU Combined figure that was advertised on Audi's website.

The advertisement said: “The A3 16 TDI is the most fuel-efficient Audi ever returning a quite remarkable 68.9mpg on a Combined cycle”. The complainant argued that the mpg figure was misleading because they had not been able to achieve it.

Audi, in response, argued that official fuel consumption figures "did not give an accurate representation of the actual fuel consumption which could be expected from any particular vehicle and were provided only to enable comparisons between different vehicles or models."

Audi also said that the VCA (the designated UK Vehicle Type Approval authority) "made clear that new car fuel consumption and CO2 emissions official figures were not fully representative of real life driving conditions because of the need to maintain strict comparability of results," and that this was "well known by consumers."

The ASA said that it understood that such figures "had to be generated under test conditions" but that this was unlikely to be clear to the average consumer. It stated that Audi should have qualified the figure and concluded that the mpg claim breached Committee of Advertising Practice (CAP) Codes covering misleading advertising and qualification.

Audi was advised to make clear that official EU test figures were used as a guide for comparative purposes and may not reflect real driving results.

As a result of the case, all car advertisements could be forced to carry a similar disclaimer.

An ASA spokesman said: “This is a significant ruling that draws a line in the sand. It is not just about Audi. It sets a precedent that will have ramifications for other car manufacturers.

“The ruling sets out quite clearly that qualification is needed when quoting mpg figures. There will be an industry-wide communication to manufacturers and trade associations so they are aware of what to expect in future.”

The Society of Motor Manufacturers and Traders (SMMT) said: “It is important that motorists understand the difference between mpg results achieved through real-world driving and the official EU laboratory tests, which benchmarks models for comparison.

"Vehicle performance depends on a range of factors including driving style, maintenance standards, climatic conditions and loads carried, so moves that enable motorists to understand these differences are welcomed by industry.”